1. Field of the Invention
This invention relates to the sizing of paper.
2. Description of the Background
Sizing in the paper industry is a process whereby a material is incorporated into the paper to render the paper more resistant to penetration by liquids, especially water.
The size may be added to the stock of the aqueous pulp use to form the paper or the formed dry paper may be passed through a solution of the size.
In North America the most popular sizing processes used acid materials and operate at an acidic pH of 4 to 5.
Sizing processes which operate in a non-acidic pH range of 7 to 8 account for about 25% of the paper and paperboard market.
A particular advantage of neutral or alkaline sizing in papermaking, is that calcium carbonate can be used as filler in place of the more expensive titanium dioxide and clay fillers used in acidic sizing. In 1992 the cost of calcium carbonate filler is about 10% that of titanium dioxide filler and about 65% that of clay filler.
A further advantage in employing calcium carbonate filler is that calcium carbonate in the paper is a source of alkalinity which provides resistance to acidic ambient conditions, and this provides longer shelf life. Furthermore, non-acidic sizing causes less corrosion in the paper machines.
In the neutral or alkaline sizing process where alkylketene dimers are employed as sizing agents, the reaction between alkylketene dimer and cellulose proceeds at a slow rate.